The present invention relates to a thermoplastic elastomer comprising a crystalline polyolefin resin and a cross-linked rubber component as well as to a process for producing such thermoplastic elastomer. More specifically, the invention relates to a thermoplastic elastomer exhibiting improved resistance to oils together with almost no tendency to deposit gummy crust onto surfaces of an extrusion die upon extrusion molding of articles therefrom and to a process for producing such thermoplastic elastomer.
Due to light weight and ease in recycling, thermoplastic elastomers based on polyolefin have found wide applications for use as energy-sparing and resources-sparing elastomers, in particular, as substitute for vulcanized rubber for parts and elements in a variety of applications such as automobiles, industrial machines and electric and electronic devices, and for architectural materials.
However, thermoplastic polyolefinic elastomers of the prior art are inferior in the resistance to oils and, in particular, suffer from a problem that they may swell upon contact with non-polar solvents, such as aromatic organic solvents, gasoline and mineral oils, resulting in limitation of the application range. The conventional thermoplastic elastomers exhibit more tendency to cause deposition of gummy crust at and around the extrusion die upon extrusion molding of such elastomers into articles than other commonly employed resins, whereby the molded articles may suffer from deterioration of appearance due to adhesion of the separated gummy crust.
An object of the present invention is to provide a thermoplastic elastomer based on polyolefin, which exhibits an improved resistance to oils and has scarce tendency to cause deposition of gummy crust onto extrusion die upon extrusion molding of articles therefrom.
Another object of the present invention is to provide a process for producing the above-mentioned improved thermoplastic elastomer based on polyolefin.
The above objects of the present invention are attained by the polyolefinic thermoplastic elastomer and the process for producing such elastomer as given below:
(1) A thermoplastic elastomer comprising (A) a crystalline polyolefin resin and (B) a cross-linked rubber component, which has
a) a gel content expressed as a cyclohexane-insoluble matter after having been soaked in cyclohexane at 23xc2x0 C. for 48 hours of at least 20% by weight,
b) a weight change range xcex94W after having been soaked in paraffin oil at 50xc2x0 C. for 24 hours amounting to 80% by weight or lower and
c) a rate of gummy crust deposition of 30 mg or less within an interval of 10 minutes occuring around the extrusion die upon extrusion of the elastomer into a ribbon on a monoaxial extruder provided with a full-flight extrusion screw having a diameter of 50 mm, an L/D ratio of 28 and a compression ratio of 4.0 through an extrusion die having an extrusion aperture of 25 mmxc3x971 mm under a gradient temperature elevation from the extruder supply inlet to the die outlet of 160-210xc2x0 C. at an extrusion rate of 20 m/min.
(2) A process for producing a thermoplastic elastomer comprising (A) a crystalline polyolefin resin and (B) a cross-linked rubber component, comprising the steps of
melt-kneading a crystalline polyolefin resin (A1) together with a rubber component (B1) in the presence of a cross linking agent (D) in a first extruder,
supplying the resulting kneaded mass in which the cross linking reaction has substantially been completed to a second extruder and
melt-kneading the so-supplied kneaded mass therein together with a crystalline polyolefin resin (A2) and/or a rubber component (B2) each supplied to the second extruder.
(3) A process for producing a thermoplastic elastomer comprising (A) a crystalline polyolefin resin and (B) a cross-linked rubber component, comprising the steps of
melt-kneading a crystalline polyolefin resin (A1) together with a rubber component (B1) in the presence of a cross linking agent (D) in a first extruder,
supplying a kneaded mass of a crystalline polyolefin resin (A2) and/or a rubber component (B2) extruded from a second extruder to the first extruder at a position within the range of the exit-side half of the first extruder and
melt-kneading the so-supplied kneaded mass (A2 and/or B2) therein together with the kneaded mass which has been subjected to the melt-kneading in the first extruder and in which the cross linking reaction has substantially been completed.